This invention relates to hydraulic control means for pipe thrust-jacking apparatus.
Pipe thrust-jacking apparatus is used for laying pipelines, particularly supply and discharge lines, in the ground. Such a pipeline is constituted by pipes made, for example, of steel or steel and concrete, and is advanced in sections from a trench. For this purpose the trench is provided with a main jacking station. Typically, such a station has up to eight hydraulic advance rams which are braced against a rear abutment, and are coupled to a pressure ring. The pressure ring acts on the rear end face of the most recently laid pipe section, and uniformly reduces and distributes the thrust forces applied by the rams to the pipeline so that strictly localised loading is avoided.
The rams are often designed as double-telescopic rams, in which case they have working strokes of considerable lengths. Difficulties arise when these rams are retracted, since the heavy pressure ring retracted therewith tends to tilt if the rams are not retracted in synchronism.
Usually, it is simply left to the skill of the operator to retract the pressure ring with the least possible tilting thereof, use being made of a relatively complicated levering technique or operating keyboard on the control platform. Not only is this manual retraction of the pressure ring very time-consuming, but faulty manoeuvring also often leads to interference with the entire pipe-advancing operation.
The aim of the invention is to provide hydraulic control means for pipe thrust-jacking apparatus which, irrespective of the skill of the operating crew, permits the pressure ring to be advanced and retracted without being tilted.